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Energy could be moved over long distances by quantum teleportation,
according to calculations done by a team of physicists in Japan. While
energy teleportation is not a new concept, it had been thought that the
amount of energy that could be sent dropped rapidly beyond short
distances. The new proposal removes this shortcoming, allowing energy to
be transferred much farther. The team believes that the theory could be
verified in a semiconductor device and that similar energy
teleportation could have occurred in the early universe.
Quantum teleportation is a remarkable idea that was first proposed by
IBM's Charles Bennett and colleagues in 1992. It involves two parties,
usually called Alice and Bob, who "teleport" a quantum state between
each other. The scheme allows Alice to send information about an unknown
quantum state to Bob, who is then able to construct a perfect copy of
that state. To do so, the pair exchange classical information while
sharing particles that are entangled quantum mechanically with each
other. Physicists have since been able to teleport atomic states over
distances of several metres and photon states over distances greater
than 100 km.
While this formulation of quantum teleportation does not provide a means to exchange energy, in 2008 Masahiro Hotta
of Tohoku University unveiled a theory explaining how energy could be
teleported. In Hotta's formulation, Alice sends Bob the information that
he needs to extract energy from the vacuum. This extraction is possible
because in quantum field theory the vacuum is not devoid of energy but
contains virtual particles that continually bubble-up and then vanish.

Entangled vacuum

Hotta's idea arises from the fact that nearby points in the quantum
vacuum are entangled. This means that if Alice and Bob are close to each
other, then Alice should be able to make a measurement of her local
field and use the result of her measurement to gain information about
Bob's local field. If Alice then passes this information to Bob through a
classical channel (for example by calling him on the telephone), Bob
can use the information to devise a strategy for extracting energy from
his local field. This energy will always be less than the energy Alice
expended in making her initial measurement. Thermodynamically, this
means that Alice can "teleport" energy to Bob in the form of the
information he needs to extract energy from the quantum vacuum.
Unfortunately, the degree of entanglement between Alice's and Bob's
local fields decays rapidly with distance. Indeed, the fraction of
Alice's energy input that Bob can recover is inversely proportional to
the sixth power of their separation. As a result, the exchange of
significant amounts of energy across meaningful distances would be
extremely difficult in practice.
In this latest work, Hotta and colleagues at Tohoku University propose a
way round this limitation by using squeezed vacuum states. These states
are identical to the vacuum state everywhere except in the region
between Alice and Bob, where the energy density is much higher. The
result is that entanglement can be maintained over much larger
distances. Indeed, if an appropriate squeezed state is chosen, Alice and
Bob's local quantum states can remain entangled across an arbitrarily
large distance.

Quantum Hall states

The researchers propose that such squeezed states could be generated
in the laboratory by suddenly expanding the length of the edge path
travelled by electrons in a quantum Hall state. The quantum Hall effect
is seen in thin semiconductors – essentially 2D sheets – that are
exposed to a strong magnetic field. Electrons in a quantum Hall state
flow unimpeded in one direction along the edge of the semiconductor and
provide a "quantum correlation channel" in which entanglement occurs.
Hotta says he is currently working with team member Go Yusa to create
such a system in the lab.
Hotta and colleagues also point out that squeezed states might have
occurred early in the history of the universe when the cosmos underwent a
brief period of rapid expansion, dubbed inflation. Quantum information
expert Renato Renner
of ETH Zurich is open to the idea that such a squeezed state might have
been created during cosmological inflation. He is not convinced,
however, that the phenomenon could be applied to the development of
quantum electronic devices. He points out that energy must be expended
to create squeezed states, which could make practical applications
difficult to achieve.
The work is described in Physical Review A and a preprint is available on arXiv.